Gabby Petito's parents are suing Brian Laundrie's parents, claiming he told them he had killed their daughter.
The lawsuit was filed on Thursday in Sarasota County, Florida, with the Petito family alleging Laundrie told his parents on 28 August last year that he had killed his fiancee, Ms Petito.
The 22-year-old was travelling across the US with Laundrie when she went missing, sparking a nationwide search.
Her body was found on 19 September in a Wyoming national park, with Laundrie as the main suspect.
The pair previously had domestic quarrels and the FBI concluded Laundrie strangled Ms Petito.
'Shocking' And 'Atrocious' Conduct
Laundrie's remains were found on 5th October and officials concluded he fatally shot himself in Carlton Reserve swamp after returning home alone on 1st September.
The lawsuit claims Christopher and Roberta Laundrie concealed that their son had confessed to them about the murder and they hid it from investigators while the search was ongoing.
"In doing so, Christopher Laundrie and Roberta Laundrie acted with malice or great indifference to the rights of Joseph Petito and Nicole Schmidt," the lawsuit claims, adding that this conduct was "shocking, atrocious, and utterly intolerable in a civilised community."
The lawsuit adds that the Laundries were "making arrangements for him to leave the country".
No evidence has been included to back up the claims and Ms Petito's parents are seeking at least $30,000 (£23,076) in damages.
The Lawsuit Is A 'Baseless' Claim
Mr and Mrs Laundrie's lawyer Steven Bertolino said his clients were fully within their rights not to talk and denied the claims made in the lawsuit.
"This lawsuit does not change the fact that the Laundries had no obligation to speak to law enforcement or any third party including the Petito family," Mr Bertolino said.
"This fundamental legal principle renders the Petitos' claims to be baseless under the law."
Ms Petito and Laundrie were both from New York but moved to his parent's home in Gulf Coast, Florida in 2019.
The couple took Ms Petito's van on their trip from Florida, which was intended to end on Halloween on the Pacific Coast.
Until the road trip and her disappearance, the families had a "cordial relationship", according to the lawsuit.
"Gabrielle Petito had hopes of becoming a travel influencer, a 'van-lifer' and document her cross-country travels" on various social media sites, the lawsuit adds.